A conventionally used ink composition for heatset offset printing is comprised of a coloring agent, a binder resin, a vegetable oil such as a drying oil, a low-boiling petroleum solvent and the like. Ink of this type is used in a form of printing in which the ink is heated after printing and dried by evaporating the low-boiling petroleum solvent in the ink (heatset web offset printing) . The drying system in heatset web offset printing has established a printing system having an extremely high productivity, normally by heating a paper face using a drying device (there exist methods such as a hot-air blowing type, a direct-heat burner type, a combination of these types, and the like) adjacent to a printing machine until the paper-face temperature becomes 100° C. or higher, forcibly evaporating the low-boiling petroleum solvent, and fixing the binder resin, the coloring agent and the like onto the print media (generally, paper).
However, raising the paper-face temperature to 100° C. or higher as described above causes problems such as wrinkles being generated on paper and high energy costs being required; to prevent these problems, there is a demand for printing at a paper-face drying temperature as low as possible. Further, in recent years, printing speed has been increased in order to improve printing efficiency, whereby the time period for printed materials to pass through the drying device has been shortened. Therefore, an ink composition for heatset offset printing capable of being dried with addition of a lower heat energy is demanded to obtain excellent printed materials under such printing conditions.
Conventionally, the content of a solvent having a low boiling point has been increased as a method for improving the drying property of the ink composition for heatset offset printing. However, the increased content of a solvent having a low boiling point has caused a problem such as lowering stability on press of the ink, thereby having limited improvement of the drying property.
Furthermore, as a method in which a form of drying other than evaporation is used, a method is known in which a drier such as metal soaps that improve the drying property of ink by oxidative polymerization, and an antioxidant such as butylated hydroxytoluene (BHT) for preventing skinning during storage of ink are used together. However, improving the drying property of ink by using this method produces more effects from the drier than those from the antioxidant, whereby occurrence of skinning is unavoidable; then, using the antioxidant to restrain occurrence of skinning to the conventional level produces more effects from the antioxidant than those from the drier, whereby the desired drying property is not possible to be achieved in this case.
Eventually, the conventionally known methods have not been capable of achieving a balance between a performance of improving the drying property and conflicting performances of maintaining stability on press and restraining skinning. Therefore, obtaining the desired drying property has been difficult without lowering storage stability and printability.